I think we’d have to be leaking hydrogen for a tad more than a century or two to notice sea level falling beyond global cooling. But! Panic!!! What are we going to do with all the oxygen?!
So, hydrogen consumption would be 10 Mton a day, sea water 100 Mton, so at 10% hydrogen loss, a ridiculously high, inefficient figure, we’d lose 10 Mton of sea water a day. Three years to lose a cubic kilometre. A billionth. 1% loss, still high, 30 years a billionth. 30 thousand years a millionth. All that oxygen!!!
We currently flare 150 billion cubic metres of gas a year. 300 Mton